React Query for data fetching in React Native

Introduction

React Query helps you get data from the internet easily and keeps it fresh. It saves you from writing lots of code to handle loading and errors.

When you want to show a list of items from a website or server in your app.

When you need to update data automatically without making the user refresh.

When you want to handle loading and error messages simply.

When you want to cache data so your app feels faster.

When you want to keep data synced in the background.

Syntax

React Native

import { useQuery } from '@tanstack/react-query';

const { data, error, isLoading } = useQuery({
  queryKey: ['key'],
  queryFn: async () => {
    const response = await fetch('https://api.example.com/data');
    if (!response.ok) {
      throw new Error('Network response was not ok');
    }
    return response.json();
  }
});

useQuery is a hook that fetches and manages data.

queryKey is a unique name for the data you want to fetch.

Examples

Fetches a list of todos from a public API.

React Native

const { data, isLoading } = useQuery({
  queryKey: ['todos'],
  queryFn: () => fetch('https://jsonplaceholder.typicode.com/todos').then(res => {
    if (!res.ok) {
      throw new Error('Network response was not ok');
    }
    return res.json();
  })
});

Fetches data for a specific user using a dynamic key.

React Native

const { data, error } = useQuery({
  queryKey: ['user', userId],
  queryFn: () => fetch(`https://api.example.com/user/${userId}`).then(res => {
    if (!res.ok) {
      throw new Error('Network response was not ok');
    }
    return res.json();
  })
});

Sample App

This app fetches a list of todos from the internet and shows them in a scrollable list. It shows a loading spinner while waiting and an error message if something goes wrong.

React Native

import React from 'react';
import { View, Text, ActivityIndicator, FlatList } from 'react-native';
import { QueryClient, QueryClientProvider, useQuery } from '@tanstack/react-query';

const queryClient = new QueryClient();

function Todos() {
  const { data, error, isLoading } = useQuery({
    queryKey: ['todos'],
    queryFn: () => fetch('https://jsonplaceholder.typicode.com/todos').then(res => {
      if (!res.ok) {
        throw new Error('Network response was not ok');
      }
      return res.json();
    })
  });

  if (isLoading) return <ActivityIndicator size="large" />;
  if (error) return <Text>Error loading todos.</Text>;

  return (
    <FlatList
      data={data}
      keyExtractor={item => item.id.toString()}
      renderItem={({ item }) => <Text>{item.title}</Text>}
    />
  );
}

export default function App() {
  return (
    <QueryClientProvider client={queryClient}>
      <View style={{ flex: 1, padding: 20, marginTop: 50 }}>
        <Text style={{ fontSize: 24, marginBottom: 10 }}>Todo List</Text>
        <Todos />
      </View>
    </QueryClientProvider>
  );
}

Important Notes

React Query automatically caches data to make your app faster.

It handles loading and error states so you don't have to write extra code.

Always wrap your app with QueryClientProvider to use React Query.

Summary

React Query makes fetching and managing server data easy in React Native apps.

Use useQuery with a unique key and a function that fetches data.

It helps with loading, errors, caching, and updating data automatically.

Practice

1. What is the main purpose of using useQuery in React Query for React Native apps?

easy

A. To style components dynamically

B. To fetch and manage server data easily with caching and updates

C. To navigate between screens

D. To handle local state only

Solution

1. Step 1: Understand the role of useQuery

   useQuery is designed to fetch data from a server and manage it efficiently in React apps.

2. Step 2: Identify its features

   It automatically handles loading states, errors, caching, and data updates, making server data management easier.

3. Final Answer:

   To fetch and manage server data easily with caching and updates -> Option B

4. Quick Check:

   useQuery purpose = fetch and manage data [OK]

Hint: Remember: useQuery is for fetching and caching data [OK]

Common Mistakes:

• Confusing useQuery with navigation or styling hooks
• Thinking useQuery manages only local state
• Assuming useQuery is for UI rendering only

2. Which of the following is the correct syntax to use useQuery to fetch data with a key 'todos' and a fetch function fetchTodos?

easy

A. const { data } = useQuery('todos', fetchTodos);

B. const data = useQuery(fetchTodos, 'todos');

C. useQuery(fetchTodos, { key: 'todos' });

D. const { data } = useQuery({ key: 'todos', fetch: fetchTodos });

Solution

1. Step 1: Recall the useQuery syntax

   The correct syntax is useQuery(key, fetchFunction), where the first argument is a unique key and the second is the function to fetch data.

2. Step 2: Match the options

   const { data } = useQuery('todos', fetchTodos); matches this syntax exactly: useQuery('todos', fetchTodos).

3. Final Answer:

   const { data } = useQuery('todos', fetchTodos); -> Option A

4. Quick Check:

   useQuery(key, fetchFn) = const { data } = useQuery('todos', fetchTodos); [OK]

Hint: useQuery takes key first, then fetch function [OK]

Common Mistakes:

• Swapping the order of arguments
• Passing an object instead of separate arguments
• Not destructuring the returned data

3. Given this code snippet:

const fetchUser = async () => {
  return { name: 'Alice', age: 30 };
};

const { data, isLoading, error } = useQuery('user', fetchUser);

if (isLoading) return <Text>Loading...</Text>;
if (error) return <Text>Error!</Text>;

return <Text>{data.name} is {data.age} years old.</Text>;

What will be displayed after the data loads successfully?

medium

A. Loading...

B. Error!

C. Alice is 30 years old.

D. undefined is undefined years old.

Solution

1. Step 1: Analyze the fetch function and query

   The fetchUser function returns an object with name 'Alice' and age 30. useQuery fetches this data with key 'user'.

2. Step 2: Understand the rendering logic

   While loading, it shows 'Loading...'. On error, it shows 'Error!'. After successful fetch, it renders the text with data values.

3. Final Answer:

   Alice is 30 years old. -> Option C

4. Quick Check:

   Data loaded = Alice is 30 years old [OK]

Hint: Check loading and error before rendering data [OK]

Common Mistakes:

• Ignoring loading or error states
• Accessing data before it loads
• Assuming data is undefined

4. Identify the error in this React Query usage:

const { data, isLoading } = useQuery(fetchPosts, 'posts');

medium

A. The arguments to useQuery are reversed; key should come first

B. useQuery requires a third argument for options

C. Destructuring data and isLoading is not allowed

D. fetchPosts should be a string, not a function

Solution

1. Step 1: Check the argument order for useQuery

   The correct order is useQuery(key, fetchFunction). Here, the fetch function is passed first, then the key.

2. Step 2: Identify the mistake

   The arguments are reversed, which will cause an error or unexpected behavior.

3. Final Answer:

   The arguments to useQuery are reversed; key should come first -> Option A

4. Quick Check:

   useQuery(key, fetchFn) order matters [OK]

Hint: Key always comes before fetch function in useQuery [OK]

Common Mistakes:

• Swapping key and fetch function arguments
• Forgetting to pass a unique key
• Assuming fetch function can be a string

5. You want to fetch a list of posts and automatically refetch the data every 10 seconds using React Query. Which code snippet correctly implements this behavior?

hard

A. const { data } = useQuery('posts', fetchPosts, { staleTime: 10000 });

B. const { data } = useQuery('posts', fetchPosts, { refetchOnWindowFocus: true });

C. const { data } = useQuery('posts', fetchPosts, { cacheTime: 10000 });

D. const { data } = useQuery('posts', fetchPosts, { refetchInterval: 10000 });

Solution

1. Step 1: Understand refetching options in React Query

   To refetch data automatically at intervals, use the refetchInterval option with the interval time in milliseconds.

2. Step 2: Match the correct option

   const { data } = useQuery('posts', fetchPosts, { refetchInterval: 10000 }); uses refetchInterval: 10000 which means refetch every 10 seconds. Other options control different behaviors.

3. Final Answer:

   const { data } = useQuery('posts', fetchPosts, { refetchInterval: 10000 }); -> Option D

4. Quick Check:

   Auto refetch = refetchInterval option [OK]

Hint: Use refetchInterval to auto-refresh data periodically [OK]

Common Mistakes:

• Confusing refetchOnWindowFocus with interval refetch
• Using cacheTime or staleTime incorrectly
• Not passing options as third argument